This invention relates to a method of continuous annealing of moving steel strips, comprising the following sequential steps: heating a steel strip to annealing temperature; maintaining the annealing temperature; first slow quenching of the steel strip; second rapid or semi-rapid quenching of the steel strip; heating the steel strip to overaging temperature; maintaining said overaging temperature; and final cooling of the steel strip.
This invention relates particularly to the rapid or semi-rapid quenching step between the annealing treatment and the subsequent overaging treatment.
As is known, the speed of the said quenching treatment affects the mechanical characteristics of a steel strip, due to metallurgical changes occurring in the steel as a function of the quenching rapidity. Therefore, in view of the broad range of desired metallurgical effects, it is very important to be able to change the speed within sufficiently wide limits.
It is therefore the object of the present invention to control and to vary the rapid or semi-rapid quenching speed of a steel strip within a very ample range, and for instance between 650.degree. C. and 50.degree. C. per second, by using the same cooling equipment and by concurrently eliminating the formation of oxidized layers on the strip.
To this end, according to the invention, the rapid or semi-rapid quenching of the steel strip is performed in an electrolytic pickling bath, in which the steel strip acts first as a cathode and subsequently as an anode, whilst the current density applied to the steel strip, when acting as a cathode, is regulated so as to control the amount of hydrogen which is developed on the surface of the steel strip and therefore so as to correspondingly control the quenching speed of the said steel strip.
The present invention is based on the acknowledgment that the hydrogen which is developed on the surface of the steel strip, when same acts as a cathode in an electrolytic pickling bath utilized as quenching bath, performs the following two actions which are in conflict with each other:
(a) A heat-insulating action with respect to the liquid of the bath, since it is less heat-conductive than the said liquid and it reduces the surface of the strip in direct contact with the liquid of the bath, thus reducing the heat exchange.
(b) A dynamic action, according to which the hydrogen developed exerts an agitation of the electrolytic bath at the boundaries of the strip, thereby enhancing the heat exchange.
We have noted that by varying the current density applied to the steel strip when same acts as a cathode in the electrolytic pickling or quenching bath, thus accordingly varying the rate of development of hydrogen on the steel strip, one of the two above-discussed actions prevails over the other. This phenomena is utilized to vary and to regulate the quenching rate of the steel strip in the electrolytic pickling bath.
Concurrently, the hydrogen which develops on the steel strip, when same is utilized first as cathode in the electrolytic pickling and quenching bath, drammatically reduces the formation of oxides. Soon after, whenever the steel strip is utilized as anode in the electrolytic pickling and quenching bath, a controlled electrolytic dissolution of the surface layer of the steel strip is performed, so as to carry out a complete surface cleaning and a thorough stabilization of the strip surface against re-oxidation. In this manner a quenched and clean steel strip is obtained.
The relation between the cooling rate of the steel strip in the electrolytic pickling and quenching bath from one side and the current density applied to said strip whenever same is acting as a cathode, and therefore the development of hydrogen on the strip itself on the other side, depend on conditions of movement of the liquid in the bath at the boundary layers of the strip and from the temperature of the bath itself.
More particularly, when the liquid of the bath has, at the layers which are adjacent to the surfaces of the strip, a laminar motion, by increasing the current density and therefore the rate of development of hydrogen on the steel strip, also the cooling rate of the strip is increased.
In fact, in the above instance the above mentioned dynamic action of the development of hydrogen prevails, that is the increased development of hydrogen promotes the agitation of the liquid layers of the bath which are adjacent to the strip and therefore it facilitates the formation of convective streams thus enhancing the heat exchange and therefore the cooling of the strip.
Instead, whenever the motion of the liquid of the bath in the layers which are adjacent to the surfaces of the steel strip is a turbolent motion, the agitation of the bath caused by the development of hydrogen on the strip acting as cathode is negligible with respect to the convective streams already present in the bath, so that when the current density and therefore the hydrogen development on the strip are increased, the heat-insulating action provided by the said hydrogen prevails, and therefore the cooling rate of the strip is reduced.
The conditions of laminar or turbulent motion in the liquid layers of the electrolytic pickling bath at the boundaries of the surfaces of the steel strip acting as cathode may be obtained by selecting a suitable feeding rate of the strip through the bath. The turbulent conditions may be also obtained by a suitable forced stirring of the electrolytic pickling bath.
The current density applied to the steel strip acting as a cathode in the electrolytic pickling and quenching bath is preferably varied in the range from 10 to 60 A/square dm in order to adjust the quenching speed of the strip. A reduction of the current density below 10 A/square dm would render difficult the pickling. An increase of the current density above 60 A/square dm would make the process too costly and not economic.
The quenching rate of the steel strip in the electrolytic pickling bath varies also with the temperature of the liquid of the bath. Particularly when the bath is at room temperature, by varying the current density on the steel strip acting as cathode between 10 and 50 A/square dm, the quenching rate of said strip may be controlled between 300.degree. and 650.degree. C./second. Conversely, when the bath is at boiling temperature, by varying the current density on the steel strip acting as cathode between 10 and 60 A/square dm, the quenching rate of said strip may be controlled between 50.degree. and 200.degree. C./second. For intermediate values of the temperature of the electrolytic quenching and pickling bath, corresponding intermediate adjustment values of the quenching rates are obtained.
The maximum limit of 50 A/square dm of the current density in the instance of room temperature of the electrolytic bath is selected in order to prevent the development of oxygen on the steel strip whenever same acts as anode. The above limitation is not valid for the ebullition temperature of the electrolytic bath, at which also with a current density of 60 A/square dm there is no development of oxygen on the steel strip acting as anode.
In the following table the presented results are those of a set of tests made on commercial type pressing steel strips having a temperature of 720.degree. C. at the entrance in the electrolytic quenching and pickling bath, formed by an aqueous solution of natrium sulphate.
______________________________________ Condition at the interface Quenching speed, .degree.C./sec, obtained in between Na.sub.2 SO.sub.4 1.2 M Na.sub.2 SO.sub.4 1.4 M strip and at room temperature at ebullition solution 10 A/dm.sup.2 47 A/dm.sup.2 10 A/dm.sup.2 60 A/dm.sup.2 ______________________________________ Laminar 400 .div. 450 500 .div. 650 80 .div. 100 150 .div. 200 Turbulent 450 .div. 550 300 .div. 450 100 .div. 150 50 .div. 80 ______________________________________